can we rely on the measurements of connected clocks?

Not all smartwatches are created equal. The least recommendable models produce absolutely fantastic readings: they overestimate the heart rate by 50% and count double the number of steps taken. But some watches – from the ranks of Garmin, Coros, Fitbit, Apple or Samsung, for example – provide more accurate measurements. Enough to help us in our quest for better shape and iron health?

Here is a snapshot of the services, all in all still quite modest, that the best models provide today. In the next few years, their abilities were to develop at the rather slow pace of their technological improvements and at the even more peaceful pace of medical studies. They are the ones who make it possible to ascertain their strengths and identify their weaknesses, which the manufacturers strive to improve.


To follow the evolution of our physical form, it is better to avoid examining the dimensions of the connected clocks too closely: their precision is very relative. They tend to overestimate the number of steps taken in a day and detect phantom steps when lying down or sitting. All you have to do is fold the laundry for five minutes without ever moving your legs, to get a hundred phantom steps added to the daily number.

Under these conditions, it is better to avoid placing too much emphasis on small differences in pace from one day to the next. Note, to reduce the number of phantom steps, that it is better to wear the watch on the non-dominant arm – the left when you are right-handed.

Does this activity figure that we have been made aware of really encourage us to improve our shape? The research is contradictory: An Australian study reports a real increase in daily strides, but a University of Florida study points out that smartwatches rarely lead to weight loss.

The key may come from a long-term follow-up, accompanied by a doctor, because the owners of connected watches sometimes tend to give them up, as the American journalist Lindsay Crouse: [A cause de ma montre], training no longer helped me push the pressure back: it increased it “, she wrote. Activity tracking can also trigger poorly experienced addictive behaviors. Not all personalities go well with watches.


Many smartwatch models provide daily estimates of calories consumed. In an ideal world, this should make it possible to match energy expenditure with the calories consumed during meals. But these estimates are wrong, according to a broad consensus in the scientific community. The measurement of the energy used is very approximate, and the knowledge of the watch users is imperfect – the amount of fat mass and the special metabolic properties vary especially from person to person.


Smartwatches are a pretty good helper for runners, but performance-seeking athletes should consider them with caution, as their accuracy is not perfect. The best watches sometimes estimate distances with a deviation of two or three percent. As long as they are equipped with a GPS, their hikes are clearly visible on the map: their route penetrates roads and rivers.

The best models are reliable enough to read the heart rate during a run at a constant pace, but during periodic training the frequency often rises twenty beats above the actual frequency. These deviations are tolerable, except for a top athlete.

Read our guide on the subject: The best connected running watches
A fourth generation smartwatch from Apple.


Sleep time recording is relatively good for people who fall asleep immediately upon contact with their pillow – except for occasional errors. But smartwatches tend to overestimate the nights of people who stay quiet for a long time before falling asleep.

Worse: their measurements are completely erroneous when they claim to distinguish between the phases of deep and paradoxical sleep – which still today require heavy equipment capable of measuring brain waves, eye movements, the amount of air inhaled, etc. Currently therefore, it is better to avoid relying on connected watches that claim to provide a sleep quality score.

In addition, research has found many cases of patients seeing these sleep measurements obsessively, which elevates their stress levels to the point that they disrupt their nights instead of improving them. An English study from the University of Oxford points to the psychological risks of a poor sleep indicator that is likely to accentuate the feeling of fatigue and lead to a negative spiral.


To date, connected watches are capable of detecting very few diseases. Their ambition focuses mainly on respiratory and cardiovascular pathologies, with a very relative utility.

This disease affects 4% of the population, it is diagnosed in only 20% of patients and can cause severe heart failure. Connected watches are more and more often equipped with an oximeter, which makes it possible to measure the oxygen droplets in the blood.

After a night of frequent falls, most watches are careful not to start an “apnea alarm”. It is up to the user to consult the nocturnal oxygen curves and interpret them, which is difficult, especially since the measurements are difficult to perform correctly and their accuracy decreases in some people. Many manufacturers insist that these measurements should not be used for medical purposes.

The Scanwatch watch from the Withings brand is more ambitious: it gives an indication of nocturnal breathing disorders (low, medium or high). In 2020, Withings also launched a study comparing the apnea detection of its Scanwatch with laboratory equipment.

However, this study disappeared in 2021 from its side, referring to the scientific work done. According to a Withings spokesman contacted by The world, the investigation is still ongoing without its date of conclusion being known. The brand indicates that it continues to develop its algorithms.

Withings Scanwatch is one of the few watches that sends an alarm in case of arrhythmia, like the Apple Watch, which also emits an alarm when the heart rate is lowered too much or accelerates too much. Unfortunately, this monitoring only makes it possible to detect a very small proportion of cardiovascular diseases, as specified in World in 2018 Philippe Steg, head of the cardiology department at Bichat hospital.

In very rare cases, these watches can save lives. But for every life they are saved, the other patients are forced to engage in dangerous treatments that are not necessarily necessary. Or worse, they can give some a false impression of protection, pushing them to ignore bodily sensations that in normal times would have made them consult for, say, a heart attack, a fairly frequent pathology.

However, according to various cardiologists, connected watches could make sense for daily monitoring of patients with known problems, especially the elderly, or patients with risk factors such as tobacco, alcohol, hypertension, diabetes. , cholesterol, obesity.

also read The new Apple Watch: a “footnote to cardiovascular prevention”
An Apple Watch, the brand that sells the most connected watches.

Does the oximetry sensor, which can detect respiratory failure, combined with other sensors found in certain watches, make it possible to diagnose Covid-19 early enough for the person to isolate themselves before becoming contagious? It is not certain, according to a Dutch study: the discovery may be too late. Smartwatches also tend to confuse Covid-19 with other diseases like the flu.

However, these clocks can be useful when the disease has broken out, to monitor oxygen levels and alert a doctor in case hospitalization seems necessary. But there are other, safer, handheld devices that measure oxygen at your fingertips that have been distributed to U.S. patients during the pandemic.

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